Geometrically optimized fast neutron detector

ABSTRACT

An improved fast neutron detector fabricated with alternating layers of hydrogenous, optically transparent, non scintillating material and scintillating material. Fast neutrons interact with the hydrogenous material generating recoil protons. The recoil protons enter the scintillating material resulting in scintillations. The detector is optically coupled to a photomultiplier tube which generates electrical pulses proportional in amplitude to the intensity of the scintillations, and therefore are an indication of the energy of the fast neutrons impinging upon the detector. Alternating layers of materials are dimensioned to optimize total efficiency of the detector, or to optimize the spectroscopy efficiency of the detector. The scintillating material is preferably ZnS, and the hydrogenous material is preferably plastic. The detector is ideally suited for well logging applications and fast neutron monitor applications.

RELATED APPLICATIONS

This application is related to application Ser. No. 09/808,216 entitled“Geometrically Optimized Fast Neutron Detector” and application Ser. No.09/808,621 entitled “Geometrically Optimized Fast Neutron Detector.”

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is directed toward an improved fast neutron detector, andmore particularly directed toward the optimization of the detectorefficiency when used in logging of earth formations penetrated by aborehole and for a variety of applications.

2. Background of the Art

In the context of this disclosure, “logging” is defined as the measureof a parameter of material penetrated by a borehole, as a function ofdepth within the borehole.

There are many types or classes of borehole logging systems. Theseclasses include, but are not limited to, electromagnetic, acoustic andnuclear systems. Each class of logging system typically comprises a“source” which emits energy into the surrounding formation, and one ormore “detectors” which measure energy returning from the formation.Detector responses, when properly analyzed and processed, yieldformation and borehole parameters of interest.

Any type or class of logging system typically comprises a source anddetector system with sufficient depth of investigation to penetrate thelogging instrument housing, penetrate the immediate borehole region,enter the surrounding earth formation, interact with the formation, andinduce some type of response which returns to the borehole and thelogging instrument to be detected and analyzed. Nuclear logging systemstypically involve the use and measure of gamma radiation and neutronradiation. These types of beta particles. As a result, nuclear logginginstruments typically comprises a source of neutrons, or a source ofgamma radiation, one or more neutron detectors, or one or more gamma raydetectors, or some combination of these different types of sources anddetectors.

Logging instruments are typically conveyed along a borehole by means ofa wireline of drill string thereby creating a “log” of formationparameters as a function of depth within the borehole. Boreholeconditions are harsh in that temperatures and pressures are high.Components within a logging instrument, such as detectors, are subjectedto these environmental conditions well as vibration and impactsresulting from the conveying of the instrument along the borehole. As anexample, nuclear detectors used in logging application must be able towithstand these harsh conditions of the borehole environment includingtemperatures which can reach 175 degrees Centigrade (°C.) or higher.

All nuclear logging systems involve the measure of statistical nuclearprocesses. As a result, statistical significance of the measurements isof prime importance since it directly affects the statistical precisionof one or more parameters of interest computed from the measurement.Statistical precision improves as the number of detector eventsincreases. It is therefore very desirable to maximize the efficiency ofnuclear detectors used in borehole logging operations. Further, space isoften limited in downhole instrumentation making it of utmost importanceto maximize detector efficiency for a given geometry allowed in thedesign of the instrument.

Attention will now be directed toward prior art neutron detectors.Liquid scintillators have been used to detect high energy or “fast”neutrons. These scintillators also respond to impinging gamma radiation.Neutron and gamma ray “events” generate different pulse shape responsesfrom liquid scintillators. Pulse shape discrimination methods thereforeprovide means for separating fast neutron and gamma ray inducedresponses in liquid scintillator detectors. Fast neutron and gamma raymeasurements can be made with a single liquid scintillator detector.Liquid scintillators are relatively efficient. Unfortunately, liquidscintillators consist of flammable mixtures, and some mixtures have verylow flash points. For these reasons, liquid scintillators are notdesirable for high temperature, high pressure downhole applications.

Gas filled detectors, such as detectors containing relatively highpressures of helium-4 (⁴He), have been used as fast neutron detectors.These detectors are relatively rugged, and can withstand relatively hightemperatures encountered within the borehole. Because the detectors aregas filled rather than liquid or solid, their detection efficiency isrelatively low, and therefore not particularly desirable for downholeapplications where statistical significance of measured detectorresponse is of prime importance.

Plastic scintillators are relatively efficient neutron detectors, ruggedin construction, and able to operate at temperatures of at least 175° C.These detectors are, however, responsive to both fast neutrons and gammaradiation. Neutron and gamma ray events can not be delineated by theshape or amplitude of pulses produced by the detector. The crystalanthracene, a hydrocarbon, is another type of solid material used infast neutron detectors but, like the plastics, can not separate fastneutron from gamma ray events using pulse shape or pulse amplitudediscrimination.

Stilbene and p-terphenyl crystals are fast neutron detectors and arereported to produce pulses which can be separated into fast neutron andgamma ray events. This class of detector does not have the flammabilityof the liquid scintillators. The crystals are, however, not rated asoperable at temperatures of 175° C. The crystals are also difficult tofabricate, and availability is questionable with the only known sourcebeing Russia.

A fast neutron detector potentially suitable for downhole applicationsis an activated zinc sulfide scintillator combined with anonscintillating plastic. The activated dopant is preferably silver (Ag)but other elements, such as copper (Cu) may be suitable or even betteractivators depending on the application of the detector. Activated zincsulfide will be denoted by the symbol “ZnS” in the remainder of thisdisclosure, with the understanding that the dopant can consist of avariety of materials. The non scintillating plastic can be any hydrogenrich material that is optically transparent and that possesses suitablemechanical properties.

Geometrically, the detector is constructed with a ZnS cylindrical coresurrounded by alternating and concentric cylinders of plastic and ZnS.The scintillator detector was first introduced by Emmerich in 1954 (W.S. Emmerich., Review of Scientific Instruments, vol. 25, page 69(1954)). Neutron and gamma ray events can be separated by pulseamplitude discrimination. Fast neutron detectors of this type areoffered commercially by the Bicron division of Saint-GobainInternational Ceramics, Inc. The material in not flammable, and it isthought that the detector can meet a 175° C. temperature rating withsome modifications. The main disadvantage of this type of detector forborehole applications is the relatively small volume, with correspondingreduction in detector efficiency. Furthermore, efficiency is notmaximized for specified detector volumes, and in particular forspecified detector geometry of diameter D and length L. Detector volumeis restricted by the lack of light transparency of ZnS, withscintillations within the ZnS element only being able to reach anoptically coupled photomultiplier (PM) tube through the transparentplastic component of the detector. The plastic component of the detectorcontains hydrogen (H). As with other H containing fast neutrondetectors, the material responds to fast neutrons impinging upon thedetector by the proton recoil process, with recoil protons generatingscintillations within the ZnS component of the detector. Detectorresponse is further enhanced by a threshold (n,p) reaction with ³²S asreported by Birks (J. B. Birks, The Theory and Practice of ScintillationCounting, Pergamon Press, page 548, Oxford, 1964). This reactionintroduces additional neutron induced proton flux within the ZnSscintillation material thereby increasing the efficiency of thedetector.

Measures of fast neutrons are used in many prior art well loggingsystems to determine formation and borehole parameters of interest. Inthese prior art systems, fast neutron fluxes are typically measuredinefficiently, and in many cases are determined indirectly in that theother parameters are measured and used to compute fast neutron fluxes.

The prior art contains patents teaching various apparatus and method formeasuring and applying neutron and gamma ray measurements to obtainparameters of earth formations penetrated by a borehole. Patents thoughtto be the most relevant to this disclosure are summarized as follows:

U.S. Pat. No. 4,122,339 to Harry D. Smith, Jr. et al discloses a loggingsystem that irradiates, with fast neutrons, earth formations penetratedby a borehole. Fast neutron population is measured indirectly frominelastic scatter gamma radiation detected with a gamma ray detectorduring bursts of fast neutrons from a pulsed neutron source. Anepithermal neutron detector is used to measure epithermal neutronpopulation following each neutron burst. The inelastic scatter gamma raymeasurement is then combined with a fast neutron/epithermal neutronratio to determine formation porosity.

U.S. Pat. No. 4,122,340 to Harry D. Smith, Jr. et al discloses a loggingsystem using epithermal and fast neutron detectors. A stilbenescintillation crystal is used to detect fast neutrons. Measurements offast and epithermal neutrons are combined to determine formationporosity.

U.S. Pat. No. 4,134,011 to Harry D. Smith, Jr. et al discloses a loggingsystem comprising one epithermal and one fast neutron detector.Formation porosity is determined by making a dual spaced fast toepithermal neutron measurement using a continuous source of fastneutrons. Stilbene is used in the fast neutron detector with a spacingfrom the neutron source of 40 to 80 centimeters (cm). Pulse shapediscrimination is used to separate gamma ray events from fast neutronevents.

U.S. Pat. No. 4,152,590 to Harry D. Smith, Jr. et al discloses a loggingsystem which is very similar to the system disclosed in U.S. Pat. No.4,134,011 summarized above. A thermal decay rate measurement is added.

U.S. Pat. No. 4,605,854 to Harry D. Smith, Jr. disclosed a loggingsystem wherein earth formation is irradiated with fast neutrons. Asingle fast neutron detector is used to measure a resulting neutronenergy spectrum by an unfolding process. The patent does not disclosespecific detector type, and whether or not gamma ray discrimination isachieved.

U.S. Pat. No. 4,631,405 to Harry D. Smith, Jr. discloses a dual spacedfast/epithermal neutron porosity logging system. Fast neutrons aremeasured at a short spacing with respect to a fast neutron source, andepithermal neutrons are measured at a long spacing with respect to theneutron source. Measurements are combined to obtain formation porosity.

U.S. Pat. No. 5,068,532 to Malcolm R. Wormald et al discloses a systemwherein fast neutrons are detected for the purpose of providingcoincident-timing information in lieu of using a pulsed neutron source.The detector is not used to produce borehole logging information,although logging is mentioned in one application.

U.S. Pat. No. 5,008,067 to John B. Czirr discloses a method formonitoring the output of fast neutrons from a neutron source element ofa well logging apparatus. The detector comprises a scintillatorcontaining oxygen. The ¹⁶O(n,p)¹⁶N reaction induced by 14 MeV neutronsproduces delayed and very large amplitude pulses resulting from the sumof detected beta-decay energy and the 6-7 MeV gamma radiation from thedecay of ¹⁶N. These pulses can be separated from other neutron and gammaray pulses.

U.S. Pat. No. 6,207,953, assigned to the assignee of the presentapplication, discloses a logging system in which fast neutrons andinelastic scatter gamma rays are measured and combined to determineformation porosity (and therefore density), and also combined todetermine formation liquid saturation. A liquid scintillator isidentified for fast neutron detection, providing both fast neutron andinelastic gamma ray counts by pulse shape discrimination. An alternateplastic scintillator and gamma ray detector combination is also taughtin the event that a liquid scintillator is not suitable for a particularapplication. Fast neutron energies are distinguished by use of pulseheight discrimination to provide borehole size compensation for airfilled boreholes.

In view of the above discussion of prior art, it is apparent that animproved detector for directly measuring fast neutron fluxes in harshborehole environments is needed. Furthermore, it is apparent that a fastneutron detector with efficiency maximized for a given detector geometryis also needed. This disclosure addresses both of these needs.

SUMMARY OF THE INVENTION

A geometrically optimized fast neutron detector is fabricated ofalternating regions of non scintillating, hydrogenous, opticallytransparent material and scintillation material. The interfaces betweenalternating regions are critical to the detector's fast neutronresponse. One geometry comprises alternating, concentric, rightcylinders of activated ZnS scintillator material and non scintillatingplastic. The ZnS activator can be Ag or Cu or any other suitableactivator. Again, the symbol ZnS is used to denote activated zincsulfide, which can be activated with a variety of dopants. The detector,however, is not limited to cylindrical geometry and may utilizealternate types of scintillator material. The plastic denotes a materialthat is rich in hydrogen (H) and that is optically transparent. Fastneutrons interact with the plastic producing recoil protons which enterthe ZnS scintillation material. The ZnS material is normally potted witha binder such as epoxy, which also contains H. Therefore, some protonrecoils will also occur within the ZnS scintillator region. Protonscreate scintillations within the ZnS, and a portion of this lightescapes the ZnS, enters the transparent plastic, and is detected by aphotomultiplier (PM) tube which is optically coupled to the detector.The PM dynode string is electrically connected to pulse amplificationcircuitry. Recoil proton energy is a function of fast neutron energyimpinging upon the plastic component of the detector. The intensity ofthe scintillation is a function the energy of recoil protons enteringthe ZnS scintillation material. The amplitude of the pulse from theamplification circuitry of the PM tube is a function of the intensity ofthe scintillation. The number of output pulses is a measure of fastneutron flux, and the amplitude of the pulses is a measure of fastneutron energy. Pulse amplitude is also affected by the position atwhich the proton recoil reaction occurs within the plastic material.This effect must be considered in using the detector in fast neutronspectrometry systems, as will be discussed in more detail in asubsequent section of this disclosure.

Recoil protons have a limited range within the plastic materials. Onlyproton recoil events occurring near a plastic-ZnS interface will enterthe scintillation material and therefore create a scintillation. ZnS isnot light transparent. As a result, only proton scintillation eventsoccurring near a ZnS-plastic interface enter a transparent plasticcylindrical annuli, and are eventually detected by the PM tube andrecorded as a fast neutron event.

There is also evidence that additional proton flux is generated withinthe ZnS scintillation material by fast neutrons through the ³²S(n,p)³²Preaction. These protons also create scintillations within the ZnSmaterial.

For a given overall detector diameter D, efficiency can generally beincreased by decreasing the radial wall thickness of the ZnS and plasticcylinders, thereby increasing the ZnS-plastic surface area. If, however,the wall thickness of the plastic cylinders is decreased too much, thecylinders cease to become an efficient source of recoil protons, andfurther cease to become a scintillation “light path” to the PMphotocathode. Furthermore, if the radial wall thickness of the ZnScylinders is decreased too much, the cylinders will not scintillate allentering recoil protons. Stated another way, the ZnS and plasticcylinder wall thicknesses for maximum detector efficiency is a“trade-off”, and these dimensions must be optimized for a given detectordiameter D. Detector efficiency can also be increased by increasing thelength L of the detector. Length is also a trade-off parameter in thatexcessive length can decrease the detector's gamma ray rejectioncapability, and further decrease the efficiency of the plastic annuli aslight paths to the PM photocathode.

The geometrically optimized ZnS/plastic fast neutron detector is ideallysuited for use in any downhole instrument in which a measure of fastneutrons is desired. One application is disclosed in the previouslyreferenced U.S. Pat. No. 6,207,953, assigned to the assignee of thepresent application, and hereby incorporated in this disclosure byreference. The logging system uses measures of fast neutrons andinelastic scatter gamma rays, which are combined to determine formationporosity (and therefore density), and also combined to determineformation liquid saturation. A pulsed neutron generator provides asource of fast neutrons. Sodium iodide is a suitable inelastic gamma raydetector, wherein the detector is wrapped with a thermal neutronabsorbing material such as cadmium to prevent neutron activation of thecrystal. A ZnS/plastic detector is used to measure fast neutrons,wherein the geometry of the detector is optimized for maximum efficiencyfor the space available for the detector within the instrument orlogging “tool”. A ratio of fast neutron energies is determined by use ofpulse height discrimination to provide borehole size compensation forair filled boreholes. As mentioned previously, both impinging fastneutron energy and the position at which a neutron induced proton recoilevent occurs within the plastic component of the detector affectmeasured pulse amplitude. It is necessary to account for proton energyloss, commonly referred to as “dE/dx”, as the proton moves from theplastic component into the scintillation component, as will be discussedsubsequently.

The geometrically optimized detector is suited for use as a monitor ofoutput from fast neutron sources. This application is not onlyapplicable to well logging apparatus and methods, but also applicable toa wide range of analytical and testing methods and apparatus which usefast neutron sources.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features, advantages andobjects of the present invention are attained can be understood indetail, a more particular description of the invention, brieflysummarized above, may be had by reference to the embodiments thereofwhich are illustrated in the appended drawings. It is to be noted,however, that the appended drawings illustrate only typical embodimentsof the invention and are therefore not to be considered limiting of itsscope, for the invention may admit to other equally effectiveembodiments.

FIG. 1a is a cross sectional view of a prior art ZnS/plastic fastneutron detector;

FIG. 1b is a side sectional view of a prior art ZnS/plastic fast neutrondetector;

FIG. 2a is a plot of pulse height from a ZnS/plastic fast neutrondetector as a function of time, wherein the pulses are induced byreactions from a pulse of 14 MeV neutrons;

FIG. 2b is a plot of 14 MeV neutron output from a neutron generator as afunction of time;

FIG. 3a is a conceptual illustration of the effects of ZnS-plasticsurface contact area upon the efficiency of a ZnS/plastic fast neutrondetector;

FIG. 3b is a conceptual illustration of the effects of wall thickness ofalternating, concentric plastic cylinders upon the efficiency of aZnS/plastic fast neutron detector;

FIG. 3c is a conceptual illustration of the effects of detector lengthupon the efficiency of a ZnS/plastic fast neutron detector;

FIG. 4a is a cross sectional view of a concentric, coaxial,geometrically optimized ZnS/plastic fast neutron detector;

FIG. 4b is a side sectional view of a concentric, coaxial, geometricallyoptimized ZnS/plastic fast neutron detector;

FIG. 4c is a perspective view of an axially layered, geometricallyoptimized ZnS/plastic fast neutron detector;

FIG. 4d is a perspective view of an axial bar, geometrically optimizedZnS/plastic fast neutron detector;

FIG. 4e is a perspective view of an axial rod, geometrically optimizedZnS/plastic fast neutron detector;

FIG. 5 is an illustration of a geometrically optimized ZnS/plastic fastneutron detector embodied in a nuclear well logging system;

FIG. 6 illustrates amplification, gain control and gamma ray rejectiondiscriminator components of the electronics section of the well loggingsystem;

FIG. 7 is a conceptual illustration of directional detector sensitivityof the geometrically optimized detector; and

FIG. 8 is a functional diagram of a fast neutron monitoring system usingthe geometrically optimized detector.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A fast neutron detector suitable for use in borehole logging instrumentsmust detect neutrons efficiently in the energy ranges from about 1million electron volts (MeV) to 14 MeV, reject any response toaccompanying radiation, be mechanically rugged, and operate at atemperature of about 175° C. The combination of separate regions ofactivated ZnS scintillator and hydrogen rich plastic produces a suitablefast neutron detector when optically coupled to a photomultiplier (PM)tube. Energetic protons are produced by fast neutrons scattering fromhydrogen within the plastic. A scintillation photon is produced when anenergetic proton reaches the ZnS scintillator. Because the linear rangeof energetic protons is very short in both the scintillator and plasticmaterials, and because light must escape the optically cloudy ZnS regionto be detected by the PM tube, the interface between the two materialsis critical to the detector's fast neutron response. Therefore, thetotal surface area of the ZnS-plastic interface is an important designconsideration. Other important design considerations are the volume ofthe detector, the detector diameter and length, the geometric shapes ofthe regions of scintillator and plastic material, the operatingtemperature that should be at least 175° C., and the manufacturabilityof the most efficient geometric configuration of the detector.

Detectors consisting of a combination of ZnS and plastic regions haveshown excellent gamma radiation rejection on the basis of pulse height.That is, the PM tube pulses produced by fast neutrons in the desiredenergy range are much larger in amplitude than pulses induced by gammaradiation with similar energy.

As will be illustrated in a subsequent section of this disclosure, gammaray induced pulses are sufficiently small in amplitude so that adiscriminator circuit can be set to reject gamma ray pulses with littleloss in fast neutron response. ZnS alone has a measurable fast neutronresponse. However, when used alone, ZnS is restricted to relativelysmall volumes due to lack of light transparency and the resultinginability of scintillation photons occurring within the material toreach the photocathode of an optically coupled photomultiplier (PM)tube. Larger volume, more efficient scintillation detectors can beobtained by combining ZnS with a nonscintillating, optically transparenthydrogen rich plastic. The plastic produces additional fast neutronresponse and provides an optical path for scintillation photons to thephotocathode of the optically coupled PM tube.

It should be understood that various scintillation materials can be usedin combination with the plastic component. As an example, the zincsulfide dopant can be Ag or Cu depending upon temperature requirements,physical geometry constraints, and even economic limitations. Theplastic could be replaced with another hydrogen rich and opticallytransparent material, such as RTV.

Prior Art Detectors

A prior art detector is shown as a cross sectional view in FIG. 1a andas a side sectional view in FIG. 1b. The detector is identified as awhole by the numeral 50. Geometrically, the detector is constructed witha ZnS cylindrical core or “bulls eye” 58′ surrounded by alternating andconcentric cylinders 60 of plastic and cylinders 58 of ZnS. The core 58′and alternating concentric cylinders 58 and 60 are bound together as aunit preferably by epoxy and enclosed at one end and around theperiphery within a an opaque cover 52. The second, uncovered end isoptically coupled at interface 51 to a PM tube 54 as best seen in FIG.1b.

Detectors of the type and geometry shown in FIGS. 1a and 1 b are madecommercially by the Bicron division of Saint-Gobain InternationalCeramics, Inc. More specifically, the Bicron model BC-720 scintillator'slength “L” dimension 62 is 0.625 inches (in.) or 15.88 millimeters (mm),and the diameter “D” dimension 64 1.5 inches (in.) or 38.1 millimeters(mm). Other diameters are currently available, but only with a length of0.625 in. There are two concentric ZnS cylinders and three plasticcylinders 60 arranged alternately around the ZnS core 58′. Allcomponents of the scintillator are potted in epoxy. The wall thicknesses66 and 68 of all plastic cylinders 60 and ZnS cylinders 58 are both0.125 in. (3.175 mm) in the model BC-720 scintillator. The plastic ismilled with concentric recesses. A bottom base plate 59 contacts the PMtube 54 at the interface 59. The concentric cylinders 58 of ZnS arefitted into the concentric recesses with an epoxy mixture.

Principles of Operation

As mentioned previously, the ZnS/plastic detector assembly respondsthrough the mechanism of fast neutrons interacting with hydrogen withinthe plastic cylinders to produce recoil protons. Recoil protons enterthe ZnS cylinders. Protons create scintillations within the ZnScylinders. A portion of light produced by the scintillations escapes theZnS and enters the transparent plastic cylinders and is subsequentlydetected by a photomultiplier (PM) tube that is optically coupled to thedetector. The PM photocathode is electrically connected through a dynodestring to pulse amplification circuitry. Recoil proton energy is afunction of fast neutron energy impinging upon the plastic component ofthe detector. The intensity of the scintillation is a function of theenergy of recoil protons entering the ZnS scintillation material andalso depends on the dE/dx of protons within the plastic. The amplitudeof the pulse from the amplification circuitry of the PM tube is afunction of the intensity of the scintillation. The number of outputpulses is a measure of fast neutron flux, and the amplitude of thepulses is a measure of fast neutron energy.

FIGS. 2a and 2 b illustrate the response of the fast neutron detectorresulting from irradiating earth formation or other material withseveral bursts or “pulses” of fast (14 MeV) neutrons. The response shownis a superposition of many such bursts.

FIG. 2b is a plot of neutron output (ordinate) from a fast neutronsource as a function of time T (abscissa). The burst is initiated at atime T₁ as indicated by the point 70 on the time axis, and terminated ata time T₂ as indicated by the point 71 on the time axis.

FIG. 2a shows the corresponding output from the photomultiplier tube andamplification circuit of the detector assembly. Pulse height oramplitude (ordinate) is plotted as a function of time T (abscissa).Prior to the burst initiation at T₁ there are essentially no outputpulses. Once the burst is initiated at T₁, relatively high amplitudepulses 72 are observed as a result of recoil protons inducingscintillations within the ZnS component of the detector. The pulseamplitude is a function of recoil proton energy which, in turn, is afunction of fast neutron energy impinging upon the detector. The numberof high amplitude pulses is a function of the intensity of fast neutronflux impinging upon the detector. High amplitude pulses cease to beobserved at time T₂ when the neutron burst is terminated. Smaller pulses74 are also observed during and after termination of the neutron burst.These result primarily from gamma radiation produced by the capture ofthermal neutrons after the burst. It is apparent that this gammaradiation generates pulses 74 of a much lower amplitude than pulsesgenerated by the fast neutron-recoil proton process. By setting adiscriminator level at a pulse height in excess of the amplitude forgamma radiation, for example 76, and recording pulses of amplitudegreater than this level, gamma radiation can be effectively removed fromthe detector response. The detector then responds only to fast neutronsover the range of about 0.5 to 14 MeV. The detector also exhibits noise,which is lower in amplitude than the neutron induced pulses 72. Noiserelated pulses are not shown in FIG. 2a for purposes of clarity.

As stated previously, there is also evidence that additional proton fluxis generated within the ZnS scintillation material by fast neutronsthrough the ³²S(n,p)³²P reaction. These protons also createscintillations within the ZnS material. Proton recoils may also beproduced in the scintillation material if the ZnS binder contains H.

Geometric Optimization of Detector Efficiency

The process of optimizing the ZnS-plastic combination for the detectionof fast neutrons is presented by way of example. The example uses thegeometry of the prior art detector, consisting of alternating concentriccylinders of ZnS and plastic. This geometry is shown in FIGS. 1a and 1b.

Recoil protons have a limited range within the plastic material. Onlyproton recoil events occurring near a ZnS-plastic interface will enterthe scintillation material and therefore create a scintillation. As aspecific example, consider the use of an acrylic resin such as Lucitefor the plastic cylinder components of the detector. Lucite meets therequirements of being a hydrogen rich source of recoil protons, isoptically transparent, and is suitable for moderate temperaturesencountered within a borehole environment. The range of a 5 MeV protonis 0.04 grams per square centimeter (g/cm²). For the density of 1.19grams per cubic centimeter (g/cm³) for Lucite, the linear range of the 5MeV proton is only 0.033 cm. Recall that the wall thickness of all ofthe plastic cylinders of the Bicron model BC-720 scintillator assemblyis 0.3175 cm. It is apparent, therefore, that a large portion of recoilprotons created by fast neutron reactions in the interior of the plasticcylinders have insufficient linear range to reach the nearest ZnSscintillator, create a scintillation, and therefore be detected. Statedanother way, excess plastic cylinder thickness decreases the overallefficiency of the detector assembly to fast neutrons for a givendiameter D. Efficiency can be optimized by varying the plastic cylinderwall thickness and varying the number of cylinders, but for a givenoverall detector diameter D.

ZnS is not light transparent. As a result, only proton scintillationevents occurring near a ZnS-plastic interface enter a transparentplastic cylinder and are subsequently detected by the PM tube andrecorded as a fast neutron event. As in the case of the plasticcylinders, excess ZnS cylinder wall thickness can decrease the overallefficiency of the detector to fast neutrons, and efficiency can beoptimized by optimizing ZnS cylinder wall thickness by using more butthinner cylinders for a given overall detector diameter D.

There are, however, “opposing” factors that affect detectoroptimization. Recall that the plastic cylinders serve not only as asource of hydrogen for the proton recoil reaction, but also serve aslight paths through which scintillations pass from the detector assemblyto the optically coupled PM tube. If the wall thickness of the plasticcylinders is reduced excessively, the transmission of scintillationlight is impaired.

Length of the detector can also be increased to conceptually increasedetector efficiency. Applications of the detector may limit the length.In addition, increasing length L can also increase the ratio of gammaradiation pulse amplitude to the proton recoil pulse amplitude therebymaking the rejection of gamma radiation events by pulse heightdiscrimination more difficult. Once again, increasing efficiency by thistechnique presents a trade-off.

As mentioned previously, the dE/dx effect of proton transport throughthe plastic must be considered if the detector is to be used as a fastneutron spectrometer. Conceptually, average dE/dx is reduced as thethickness 66 of the plastic cylinders are decreased. This may result inadditional alternating cylinders of plastic and ZnS for a given detectordiameter D. Such an optimization for spectroscopy reasons might decreasethe overall efficiency of the detector. Stated another way, if thedetector is to be used as a fast neutron spectrometer, optimization mustinclude both efficiency and spectroscopy considerations.

There is also evidence that additional proton flux is generated withinthe ZnS scintillation material by fast neutrons through the ³²S(n,p)³²Preaction. These protons also create scintillations within the ZnSmaterial, and the wall thickness of the cylinders determines the portionof scintillation reaching the plastic cylinder light paths.

The factors governing geometric optimization of a ZnS/plasticscintillator of given diameter D and length L are shown conceptually inFIGS. 3a, 3 b and 3 c.

FIG. 3a is a plot of an efficiency term E(SA) as a function of surfacecontact area SA between the ZnS and plastic cylinders. This contact areais, of course, governed by the number n of alternating ZnS and plasticcylinders. For a given diameter D, SA is a function of the thickness tsiof the ZnS cylinders, where (i=1, . . . , n). For reasons discussedabove, E(SA) increases with increasing SA as is illustrated by the curve80.

FIG. 3b is a plot of an efficiency term E(1/t_(pi)) as a function of theinverse of plastic cylinder wall thickness t_(pi), where again (i=1, . .. , n). For a given value of D and for reasons discussed above,E(1/t_(pi)) varies as a function of 1/t_(pi) as is illustrated by thecurve 81. E(1/t_(pi)) increases as a function of 1/t_(pi) to a certainvalue 83, and then starts to decrease as increasingly thin plasticcylinder walls impede the passage of scintillation light to the PMtube.54 (see FIG. 1b).

FIG. 3c is a plot of an efficiency term E(L) as a function of detectorlength L. For a given value of D, E(L) increases as a function of L forreasons discussed above as is illustrated by curve 82. At some value ofL, further increase in length does not increase E(L) since scintillationlight, formed in a portion of the detector opposite the PM tube, can notreach the PM tube and there be sensed. This point is conceptuallyillustrated at 84.

The total detector efficiency, E_(total), for a given detector diameterD, can be expressed mathematically as

E _(total) =f(E(SA),E(1/t _(pi)),E(L),E(A _(γ)))  (1)

where f(E(SA),E(1/t_(pi)),E(L), E(A_(γ))) is a function containing theefficiency terms E(SA), E(1/t_(pi)), and E(L). E(A_(γ)) is an efficiencyterm which accounts for the relative amplitude of gamma ray inducedpulses as a function of detector length. For a given detector diameterD, which is usually determined by the physical application of thedetector in an instrument such as a logging tool, the parameters ZnScylinder wall thickness tsi (and thus SA), plastic cylinder wallthickness (and thus1/t_(pi)), and length L are adjusted to maximize thetotal counter efficiency E_(total). If the physical application alsorestricts the length L, then only the first two parameters are adjustedto maximize E_(total). E(A_(γ)) is not an adjustable term. Theefficiency of a ZnS/plastic detector can, therefore, be customized for agiven physical dimension to geometrically optimize the detectionefficiency for fast neutrons.

The detector can also be optimized for fast spectroscopy applications byminimizing the previously discussed dE/dx energy loss of recoil protonswithin the plastic component of the detector. Another efficiency termE(dE/dx) expresses the functional dependency of the energy loss dE/dx.The detector efficiency for fast neutron spectroscopy, Espec, for agiven detector diameter D, can be expressed mathematically as

E _(spec) =f(E(SA),E(1/t _(pi)),E(L),E(A _(γ)),E(dE/dx))  (2)

The efficiency terms E(SA), E(1/t_(pi)), E(L) and E(dE/dx) are adjustedto maximize E_(spec). It should be noted that E(dE/dx) is also afunction of the plastic thickness tpi for reasons discussed above.

FIGS. 4a and 4 b illustrate end and side sectional views, respectively,of a geometrically optimized ZnS/plastic fast neutron detector 100comprising n pairs of ZnS and plastic cylinders. The axial center lineof the detector is identified by the numeral 92. The ZnS cylinders 88are denoted as s_(i) (i=1. . . . , n) starting with the core cylinder88′ as s₁. The alternating plastic cylinders are denoted as p_(i) (i=1.. . . , n). Cylinder wall thicknesses for the ZnS cylinders 88 andplastic cylinders 90 are identified as t_(si) and t_(pi), respectively,where (i=1, . . . , n). It should be noted that cylinder wallthicknesses need not be equal. That is, it is not necessary for t_(pj)to equal t_(pj+1), or t_(sj) to equal t_(sj+1), or t_(pj) to equalt_(pj), where 1<j<n. As an example, it might be advantageous tofabricate a detector where t_(si)<t_(pi) for any or all values of i=1 .. . n. Cylinder thicknesses can be adjusted in any manner to maximizethe total efficiency E_(total), or the spectroscopy efficiency E_(spec),as long as the constraints on D (identified as dimension 84) and L(identified as 86) of the detector 100 are satisfied.

The detector is encapsulated within a light-tight housing (not shown)with the exception of the end that is optically coupled to the PM tube.

It should be noted that materials used in a geometrically optimized fastneutron detector are not limited to Lucite and ZnS. The plasticcylinders can be fabricated from any material which provides a hydrogenrich source for the proton recoil reaction, which is opticallytransparent, and which can withstand high temperatures encountered inthe borehole environment. The scintillating cylinders can be fabricatedfrom any material which scintillates upon proton bombardment, and whichproduces scintillations of intensity related to the energy of thebombarding protons.

It should also be noted that the geometrically optimized detector is notlimited to a right cylindrical shape. As an example, alternatingrectangular layers of plastic and scintillating material can be used tofabricate a detector. FIG. 4b could equally represent a cross sectionalview of a rectangular detector.

Optimization methods previously described for the right, concentriccylindrical detector are also applicable to a rectangular detector, andto other geometric forms forming a right cylinder. FIG. 4c is aperspective view of a layered detector 300 comprising alternatingregions of scintillation material 302 and plastic material 304 in theform of panels. FIG. 4d is a perspective view of an axial bar detector320 comprising a grid of perpendicular layers of scintillation material322 contacting plastic material 324. FIG. 4e is a perspective view ofaxial rod detector 340 shown with a cut-away to more fully discloseconstruction. Cylinders of scintillation material 342 are dispersedaxially within a surrounding right axial cylinder of plastic material344 and, in the example shown, extend from the top to the bottom of thecylinder of plastic material. Conversely, in FIG. 4e the dispersedcylinders 342 could be plastic material and the cylinder 344 thescintillation material. Cylinders 342′ are shown in the cut-away portionof the cylinder 344 for purposes of illustration.

Detectors can also be fabricated in conic, triangular, or virtually anyshape required for a particular application. The only requirement foroperation and optimization set forth in this disclosure are (a) that thedetector comprise alternating components of plastic and scintillatormaterial, and (b) that the plastic component provides a path forscintillation light to a PM tube or an alternate device which respondsto impinging light.

Logging Application of an Optimized ZnS/Plastic Detector

FIG. 5 depicts a logging tool 21 suspended within a borehole 40 by alogging cable 24. The borehole 40 penetrates earth formation 32, and isshown cased with typically steel casing 38. The annulus defined by theoutside diameter of the casing 38 and the borehole wall 34 is filledwith cement 36. One end of the logging cable 24 is mechanically andelectrically connected to the logging tool 21 by means of a cable head23. The cable 24 then passes over an upper sheave wheel 26′ and acalibrated sheave wheel 26 and to a winch reel 31. A depth indicator 27is attached to the sheave wheel 26 thereby permitting a measurement ofthe amount of cable deployed in the borehole 24, and therefore the depthof the logging tool 21 in the borehole 40. The depth indicator iselectrically connected to a set of surface equipment 28 which is used tocontrol the operation of the logging tool 21 and to process and storedata measured by the logging tool. The surface end of the logging cableterminates in a winch 31, and is electrically connected to the surfaceequipment 28 through electrical slip rings 29. Measurements made by thelogging tool 21 are output to a recorder 30 which forms a tabulation ofthe measurements as a function of depth at which they were measuredthereby creating the previously defined “log” 33. It should beunderstood that the log 33 can be in the form of an analog plot, adigital tabulation, or even a magnetic recording such as a tape or disk.

While FIG. 5 depicts a logging tool, the optimized ZnS detector couldlikewise be used in a logging-while-drilling tool of the typeillustrated in U.S. Pat. No. 5,091,644, and conveyed within the borehole40 by means of a drill string.

Still referring to FIG. 5, the logging tool 21 comprises a pressuretight housing 22 which contains a neutron generator assembly 20, a“near” axially spaced gamma ray detector 14 and a “far” axially spacedgamma ray detector 14′ such as sodium iodide, and a “far” axiallyspaced, geometrically optimized ZnS/plastic fast neutron detector 100biased to detect fast neutrons over a range of about 0.5 MeV to 14 MeV.The neutron generator assembly 20 comprises a high energy neutronproducing tritiated target 10 with associated deuterium gas reservoirand accelerated ion beam, high voltage supplies and pulsing electronics(not shown). Neutron generators, which produce 14 MeV neutrons by thedeuterium-tritium reaction, are well known in the art and arecommercially available.

A monitor detector 42 is preferably located close to the neutron target10 in order to monitor the somewhat variable neutron output produced bythis type of generator. The monitor detector is also a geometricallyoptimized ZnS/plastic fast neutron detector. The monitor detector biasis set to measure neutrons of energy of about 12 MeV or higher in orderto more closely monitor the direct fast neutron output from the neutrongenerator 20 and reject events from neutrons that have been scattered bythe surrounding environs. Logging system response can be normalized to afixed or “standard” neutron output by means of the monitor detectorresponse. The use of the optimized detector as a fast neutron monitor inother applications will be discussed in detail in a subsequent sectionof this disclosure.

The neutron source is pulsed and is capable of variations in both thepulse repetition rate and the pulse width. The preferred embodiment fora formation porosity/gas saturation logging system employs a pulserepetition rate of 3,000 Hz with a pulse width of about 30 μsec.

Still referring to FIG. 5, a fast neutron and gamma ray shield 12 isplaced between neutron generator target 10 and the near gamma detector14. The shielding material is preferably a material such as steel whichwill scatter, rather than thermalize, fast neutrons emitted from thetarget 10 into the formation 32 so that they can interact with formationnuclei thereby providing useful parametric information. The shield 12also preferably contains an effective gamma ray shielding component toshield the gamma ray detector 14 from gamma radiation induced by theemitted neutron flux in the immediate vicinity of the target 10. Thenear detector 14 is located as close as possible to the target 10, whileallowing for an adequate axial thickness for shield 12. The spacing ofthe near detector 14 from the target 10 is about 25 cm. The near gammadetector 14 preferably consists of a scintillator coupled to aphotomultiplier tube. As stated previously, sodium iodide is an adequatescintillator for the near detector 14, although a scintillator with afaster decay time and made of a material that does not activate bythermal neutrons would be preferred. If sodium iodide is used for neardetector 14, then it must be wrapped with a thermal neutron shield suchas cadmium to minimize thermal activation within the crystal, and thegamma ray emissions resulting from the activation. In addition, theenergy response of the detector is preferably electronically biased toexclude activation produced pulses within the detector crystal.

Again referring to FIG. 5, a fast neutron and gamma shield 16 is placedbetween near detector 14 and fast neutron detector 100. This shield,like the shield 12, prevents fast neutrons from the neutron generatorassembly 20, and neutron induced gamma radiation, from “streaming”directly down the axis of the logging tool 21 and into the detector 100.If sodium iodide is used for far gamma ray detector 14′, it must also beshielded with cadmium.

An electronics package 18 controls and provides power to the variouselectronic components of the downhole instrument 21. Pulse amplitudecounting and biasing circuitry is included within the electronicspackage 18 providing the previously discussed bias levels for the fastneutron detectors 42 and 100. FIG. 6 illustrates some of the componentsincluded in the electronics package 18. Pulses from the PM tube 54 areamplified by an amplification circuit 150. Amplified pulses then passthrough a gain control circuit 152, and subsequently through adiscriminator 154 in which pulses resulting from gamma radiationimpinging upon the detector 50 are rejected.

Attention is next directed toward the analysis of the response of thefast neutron detector 100 to fast neutrons, and the gamma ray detectors14 and 14′ to both prompt and delayed gamma radiation resulting frominelastic scatter and thermal capture reactions, respectively. Detectors14, 14′ and 100 in FIG. 5 are gated so that their pulses are processedand stored during certain time intervals in reference to the beginningof each pulse of neutrons from the neutron generator 20. These timegates are designed to produce detector responses to both prompt and todelayed radiations. The fast neutron detector 100 is gated ON during theneutron burst between times T₁ and T₂ as illustrated in FIGS. 2a and 2b. The gamma ray detectors are gated ON also during the time interval T₁to T₂, during which both prompt gamma radiation and capture gammaradiation are detected. The gamma ray detectors 14 and 14′ aresubsequently gated ON for two additional time intervals after time T₂,which is the termination of the neutron burst. These two additionalgamma ray detector gates are used to measure thermal capture radiation,which is subsequently subtracted from the gamma ray count rate measuredduring the burst leaving only a measure of the desired prompt gammaradiation in the time interval T₁ to T₂. A more detailed description ofthis process is found in the previously referenced U.S. Pat. No.6,207,953.

The porosity (or density) and water or gas saturation of earthformations 32 are determined by combining measures of prompt gammaradiation made with detectors 14 and 14′ resulting from fast neutronreactions, and measures of fast neutron radiation made with detector100. Interpretation charts determined from calibration measurements withthe logging system in known formation and borehole conditions, and fromcomputer simulations, are used to infer the formation parameters ofinterest (porosity and water or gas saturation) from the measured gammaand neutron count rates. These procedures are disclosed in detail in thepreviously referenced U.S. Pat. No. 6,207,953. The interpretation isinfluenced by formation matrix composition and by borehole conditions,and will be uncertain to the extent that these other parameters are notknown. The effects of a gas filled borehole upon measures of desiredparameters of interest can be minimized with a measure fast neutrons ata high energy level and a low energy level. Referring to FIG. 2a, pulsesfalling within the amplitude range from point 76 to point 77 representfast neutrons at a low energy level. Pulses falling within the amplituderange from point 77 to 79 represent fast neutrons at a high energylevel. Combining the measures of high and low energy fast neutronsyields a borehole correction. Corrections to minimize the effects ofvariations in liquid and gas filled borehole conditions are disclosed indetail the previously referenced U.S. Pat. No. 6,207,953.

Referring again to FIG. 5, the high energy neutron source 20repetitively emits pulses or bursts of neutrons at target 10 inrelatively short duration of approximately 30 μs, with a repetition rateof about 3,000 pulses per second. The fast neutrons from target 10 reactpromptly with atomic nuclei in formation 32, principally silicon andoxygen in the case of a silica matrix, or calcium, carbon and oxygen inthe case of a limestone matrix. Some of the fast neutrons also scatterfrom the hydrogen associated with any water present in the formation. Ofcourse, the neutrons also react with nuclei in the tool housing 22 andnearby borehole casing 38, cement 36, and any liquid that might be inthe borehole 40. These latter reactions produce undesirable responses inthat they yield no information concerning the formation parameters ofinterest and further interfere with the response of interest from theformation. As discussed previously, detectors 14, 14′ and 100 are gatedon during the time interval T₁, to T₂ to detect fast neutrons and promptgamma rays during the time of the neutron burst. The prompt gammaresponse contains, as discussed earlier, an unwanted capture componentdue to the thermalization of fast neutrons and their subsequent captureduring the time of the neutron source pulse. Corrections are made toeliminate the thermal capture components.

Again referring to FIG. 5, some of the gamma rays detected promptlyduring the neutron burst, by detectors 14, 14′ and 100, are frominelastic scattering of the fast neutrons from nuclei in formation 32.Some of the fast neutrons detected by fast neutron detector 100 havescattered elastically and inelastically from nuclei in formation 32. Thedetected neutrons have an energy in the range from 0.5 to 14 MeV.Scattering from the hydrogen contained in the formation 32 has a uniqueeffect on the flux and the energy distribution of fast neutrons in thevicinity of the gamma ray detectors 14 and 14′, and fast neutrondetector 18. This is due to two properties of hydrogen which are (1) thescattering probability is substantial at the 0.5 MeV minimum detectedenergy, but decreases rapidly in the detected range from 2 to 14 MeV,and (2) the hydrogen nucleus has nearly the same mass as the neutron sothat, unlike any other element, it can acquire most or all the energyfrom a fast neutron in a single scattering. The fast neutron measurementtherefore has a special response to the hydrogen associated withmoisture or hydrogen content of the formation surrounding the borehole.For most earth formations, the rock formation does not contain hydrogen,and this special response is associated uniquely with the liquid thatfills pore space of the rock matrix. The combined effects of theformation matrix and pore liquid on the fast neutron flux lead to asmall dependence on bulk density and a much larger dependence on atomdensity. For partial or no liquid saturation of the pore space, atomdensity decreases and there can be a large change in the neutron flux.If the capture gamma ray component is removed from the prompt gamma raymeasurement, the resulting inelastic gamma ray responses from detectors14 and 14′ are primarily sensitive to changes in bulk density of theformation, which is also a weak function of the moisture content of theformation. Stated simply, the inelastic gamma and fast neutron responsesare both affected by changes in formation density (or porosity) and bychanges in formation water (or gas) saturation. However, the tworesponses depend differently on changes in these formation parameters.Therefore, for a known formation matrix such as silica or limestone, thedisclosed logging system can produce values for water (or gas)saturation and porosity (or bulk density) from the combined measuresfast neutron and inelastic gamma responses by use of a two-dimensionaltransform grid determined from calibration data with known physicalmodels and from computer simulations. A given transform grid isappropriate only for a particular rock matrix and set of boreholeconditions. This transform is discussed in more detail in previouslyreferenced U.S. Pat. No. 6,207,953.

The logging system utilizing the geometrically optimized fast neutrondetector 100 can be effectively operated in cased and open boreholes,and in gas filled and liquid filled boreholes. This does not, however,imply that the measurements are independent of borehole conditions. Theeffects of borehole conditions on the measurements can, however, bedetermined and the measurements can be compensated or corrected forthese borehole conditions as discussed in detail in the previouslyreferenced U.S. Pat. No. 6,207,953. The transform grids disclosed U.S.Pat. No. 6,207,953 produce the desired formation parameters of porosity(or bulk density) and gas saturation (or water saturation) when thesystem logs a liquid filled borehole and measures fast neutron countsversus depth, and inelastic gamma ray counts versus depth, and when theformation mineral (matrix) and borehole conditions conform to thoseassumed for the nominal transform grid. Stated another way, onetransform grid represents the response of the logging system for one setof borehole conditions and for one formation lithology. For gas filledboreholes, a transform grid representing one borehole condition can beused to process data measured under different boreholes by combininghigh energy and low energy fast neutron measurements as previouslydiscussed.

The use of the optimized fast neutron detector in a specific wirelinelogging tool has been disclosed in detail above. It should beunderstood, however, that the optimized fast neutron detector can beused in any wireline, measurement-while-drilling, or any other type oflogging tool requiring a measurement of fast neutron flux.

Application of an Optimized Plastic Detector as a Fast Neutron Monitor

As discussed previously, the disclosed logging system requires a measureof neutron generator output in order to obtain accurate measures ofparameters of interest. Many non-logging analysis and testing systemsalso use sources of fast neutrons. These systems include activationanalysis systems, mineral assay systems, industrial waste monitoringsystems and the like. Two of these systems will be discussed briefly asbackground information in the disclosure of the geometrically optimizeddetector embodied in a fast neutron monitoring system.

Fast neutron activation analysis was developed in the late 1950s.Samples of material are irradiated with fast neutrons for apredetermined length of time. Fast neutrons interact with elementswithin the system to form radioactive isotopes. These isotopessubsequently emit delayed gamma radiation of characteristic energy. Withproper calibration of the system, and with proper control of variableparameters such as irradiation time, sample geometry, detectorefficiency and the like, the measured intensity of the characteristicgamma radiation can be related to a concentration of the element ofinterest within the sample. Characteristic gamma ray intensity can alsovary as a function of the output of the fast neutron source. Neutronoutput must, therefore, be accurately monitored in order to obtaindesired parameters of interest.

A method for analyzing the grade of coal was introduced in the 1970s,wherein coal is irradiated with 14 MeV neutrons, and induced promptgamma radiation is measured to determine carbon and oxygen content ofthe coal. A measure of these elemental concentrations is then related tothe quality or “grade” of coal. The intensity of measured gammaradiation is used to determine element content. Intensity of measuredgamma radiation can, however, also vary with variations in the output ofthe neutron generator. An accurate monitor of fast neutron output fromthe source is, therefore, required to obtain accurate measures of coalgrade.

Fast neutron sources, operated in an environ other than a vacuum,produces a wide variety of other types of radiations. These radiationsare induced by the fast neutrons interacting with virtually any materialsurrounding the fast neutron source. Epithermal and thermal neutrons aregenerated by the interaction of fast neutrons with any material in thevicinity of the source. Prompt gamma radiation is generated by theinelastic scatter of fast neutrons with material in the surroundingenvirons. Thermal capture gamma radiation is induced by the capture ofthermalized neutrons by elements in the surrounding environs. Previouslydiscussed activation gamma radiation is generated by interaction offast, epithermal and thermal neutrons with elements within materialssurrounding the fast neutron source. Any fast neutron monitor detectormust, therefore, be insensitive to the varying and often intense“secondary” radiations generated by the neutron source, and respond onlyto fast neutrons emitted by the source. If the surrounding environscontains a high concentration of light elements, fast neutrons cease tobe “fast” after the first interaction with such nuclei. This isparticularly true in wellbore environments where the hydrogenconcentration per unit volume is often relatively high.

Prior art fast neutron monitor systems are relatively numerous and wellknown. Stated generically, the systems are typically based upon pulseshape discrimination, gamma ray shielding, detector timing, andcombinations of thereof. All suffer from the inability to discriminatethe effects of induced gamma radiation and the inability to discriminatethe effects of neutrons which have undergone some type of scatterreaction, but still posses relatively high energies in the MeV range.The geometrically optimized fast neutron detector minimized theseproblems found in prior art systems.

FIG. 8 is a functional diagram illustrating a fast neutron source 210such as a 14 MeV neutron generator. A geometrically optimized detector212, optically coupled to preferably a PM tube 214, is placed in closeproximity to the source 210. Pulses from the source are amplified withan amplifier circuit 216, and the gain of the system is stabilized witha gain control circuit 217. Pulses from the gain control circuit passthrough a discriminator 218 where all “events” which have undergone anytype of reaction with nuclei in the surrounding environs are rejected.The output of the discriminator circuit 218 to a counter 220 is,therefore, directly related to the output of the fast neutron source220.

Since the detector 210 is placed within close proximity of the fastneutron source 210, the neutron flux impinging upon the detector istypically quite intense. Maximization of detector total efficiency isusually not necessary. The detector is, therefore, geometricallyoptimized for spectroscopy efficiency as stated mathematically inequation (2), rather than for total efficiency as stated mathematicallyin equation (1). With the detector 212 optimized for spectral precision,the discriminator can be set relatively high. For purposes ofdiscussion, assume that the fast neutron source is a deuterium-tritiumneutron generator that produces neutrons at a nominal 14 MeV level.Neutron output is typically not truly monoenergetic, and the outputenergy is typically an angular distribution centered at approximately 14MeV. Furthermore, single scatter events usually decrease the neutronenergy well below approximately 12 MeV. Considering these twomechanisms, a detector bias set at approximately 12 MeV essentiallyrecords unperturbed fast neutron output. Prior art systems do not havethe gamma ray rejection features, and the ability to be spectrallyoptimized to allow a meaningful bias level to be set with accuracy andprecision at approximately 12 MeV.

Other Applications

The geometrically optimized fast neutron detector can be fabricated toexhibit directional neutron detection characteristics. FIG. 7 is ahighly conceptualized illustration of possible directional detectionbias properties of the geometrically optimized detector. The detector200 is shown in sectional view, and can be either cylindrical orrectangular Alternating layers of plastic 172 and scintillating material174 are shown. Assume that a neutron 180 impinges upon the detector fromthe right, travels a distance 1 ₁ identified as 190 within plasticmaterial 172, and interacts at a point 186 to form a recoil proton.Assume that a second neutron 182 impinges upon the detector from theright, travels a distance 1 ₂ identified as 192 within plastic material172, and interacts at a point 188 to form a recoil proton. Finally,assume that a third neutron 184 impinges upon the detector from theright, travels a distance 1 ₃ identified as 194 within plastic material172, and interacts at a point 189 to form a recoil proton. There is ahigh statistical probability that all three recoil protons will enterscintillating material 174, create scintillations, and thesescintillations will be sensed by an affixed PM tube (not shown). Itshould be noted that detection probability is relatively high only ifthe proton is produced close to the interface with the scintillatormaterial. Stated simply, the three neutrons 180, 182 and 184 impingingupon the detector from the right will probably be “detected”. Nextconsider the same three neutrons, now denoted by 180′, 182′ and 184′,impinging upon the detector from the top. Assuming for simplicity thatthe fast neutron attenuating properties of plastic and scintillationmaterial are the same, the neutron 180′ will (statistically) travel thedistance l₁ and interact at a point 186′ to form a recoil proton. Again,there is a high statistical probability that neutron 180′ will bedetected. If, however, neutrons 182′ and 184′ travel the samestatistical path lengths l₂ and l₃, they will pass through the detectorundetected. In summary, there is a high statistical probability thatthree neutrons entering the detector from the right will be detected.Conversely, there is a high statistical probability that only one of thethree neutrons will be detected when impinging upon the detector fromthe top. This is because proton production is “directional”, andscintillation photon production is proportional to proton production.The efficiency of the detector fabricated and oriented as shown in FIG.7 is, therefore, highly biased in the horizontal direction.

Although the geometrically optimized fast neutron detector was conceivedfor use in boreholes to log parameters such as density (porosity) andgas (water) content in subsurface geologic formations, the detector hasother applications in apparatus to analyze other materials. As anexample the ZnS/plastic fast neutron detector can be used to analyzehighway roadbeds prior to paving the surface. It is very important toestablish that this substrate material is of the proper bulk density andmoisture content before the paving process begins. The existing nucleargamma density and neutron moisture gauges used for this purpose arelosing favor with the industry because of the required isotopicradioactive sources and their related licensing and safety difficulties.Apparatus of the present invention comprising a geometrically optimizedZnS/plastic fast neutron detector, a prompt gamma ray detector, and apulsed neutron source which can safely be deactivated represents adesirable replacement for existing density and neutron gauges for thehighway surface applications. Similar applications may exist inindustrial process control where material density and/or moisturecontent must be monitored during manufacture of products.

While the foregoing disclosure is directed to the preferred embodimentsof the present invention, other and further embodiments of the inventionmay be devised without departing from the basic scope thereof, and thescope thereof is determined by the claims which follow.

What is claimed is:
 1. A logging tool for measuring a property ofmaterial penetrated by a borehole, comprising: (a) a nuclear radiationdetector comprising alternating components of scintillating material andnon scintillating, hydrogenous, optically transparent material, whereinsaid components are fabricated to optimize detector response; (b) meanscooperative with said detector to generate a signal indicative ofradiation impinging upon said detector; and (c) means for transformingsaid signal into a measure of said property; wherein (d) geometricconfiguration and dimensions of said components are selected to optimizedetector spectroscopy efficiency response.
 2. The logging tool of claim1 wherein said components comprise: (a) a plurality of axially parallelcylinders distributed within a bounding axially parallel cylinder; and(b) said scintillating material alternates with said non scintillating,hydrogeneous, optically transparent material at each interface of saidcomponents.
 3. The logging tool of claim 1 wherein said scintillatingmaterial is ZnS.
 4. The logging tool of claim 1 wherein said nonscintillating, hydrogenous, optically transparent material is plastic.5. The logging tool of claim 1 wherein said logging tool is conveyedalong said borehole by means of a wireline.
 6. The logging tool of claim1 wherein said logging tool is conveyed along said borehole by means ofa drill string.
 7. The logging tool of claim 1 further comprising a fastneutron source.
 8. The logging tool of claim 1 wherein said propertycomprises gas saturation.
 9. The logging tool of claim 1 wherein saidproperty comprises porosity.
 10. The logging tool of claim 1 whereinsaid property comprises bulk density.
 11. A method for measuring atleast one parameter of material in an environs of a borehole penetratingearth formation, the method comprising: (a) irradiating said materialwith pulses of high energy neutrons produced by a neutron generator; (b)measuring fast neutrons resulting from interaction of said high energyneutrons with said material using a geometrically optimized ZnS/plasticfast neutron detector; (c) using said measure of fast neutrons indetermining said parameter of said material; (d) measuring prompt gammaradiation resulting from inelastic scatter of said high energy neutronswithin said material and combining with said measure of fast neutrons indetermining said parameter of said material; (e) providing a firstaxially spaced gamma ray detector within said borehole at a firstspacing from said neutron generator and providing a second axiallyspaced gamma ray detector at a second spacing from said neutrongenerator; (f) measuring said prompt gamma radiation at said first andsaid second spacings with said gamma ray detectors; (g) axially spacingwithin said borehole said geometrically optimized ZnS/plastic fastneutron detector at a third spacing from said neutron source; and (h)measuring said fast neutrons at said third spacing with saidgeometrically optimized ZnS/plastic fast neutron detector; wherein; (i)said gamma ray measurements and said fast neutron measurements at a highenergy level and a low energy level are made during said neutron pulses.12. The method of claim 11 comprising the additional step of measuringduring said neutron pulses prompt gamma radiation resulting frominelastic scatter of said high energy neutrons within said material andcombining with said measure of fast neutrons to determine said parameterof said material.
 13. The method of claim 11 wherein said second spacingand said third spacing are greater than said first spacing.
 14. Themethod of claim 11 further comprising the step of combining said fastneutron high energy measurement and said fast neutron low energymeasurement to minimize effects of said borehole upon said determinationof said parameter of said earth formation.
 15. The method of claim 11wherein said at least one parameter comprises gas saturation.
 16. Themethod of claim 11 wherein said at least one parameter comprisesporosity.
 17. The method of claim 11 wherein said at least one parametercomprises bulk density.
 18. The method of claim 11 comprising theadditional step of energy discriminating a response of saidgeometrically optimized ZnS/plastic fast neutron detector so that saiddetector responds only to fast neutrons impinging thereon.
 19. A methodfor measuring a property of material penetrated by a borehole,comprising: (a) placing a nuclear radiation detector within saidmaterial, wherein (i) said detector comprises alternating components ofscintillating material and non scintillating, hydrogenous, opticallytransparent material, and (ii) said components are dimensioned tooptimize detector efficiency; (b) providing means cooperative with saiddetector to generate a signal indicative of radiation impinging uponsaid detector; (c) providing means for transforming said signal into ameasure of said property; and (d) selecting geometric configuration anddimensions of said components to optimize spectroscopy efficiency of thedetector for a specified detector diameter.
 20. The method of claim 19wherein said components comprise: (a) a plurality of axially parallelcylinders distributed within a bounding axially parallel cylinderthereby forming interfaces; and (b) said scintillating materialalternates with said non scintillating, hydrogeneous, opticallytransparent material at each said interfaces.
 21. The method of claim 19wherein said scintillating material is ZnS.
 22. The method of claim 19wherein said non scintillating, hydrogenous, optically transparentmaterial is plastic.
 23. The method of claim 19 further comprising: (a)mounting said detector within a pressure housing; and (b) generatingsaid signal from an output of said detector which is indicative of a lowenergy level and a high energy level of radiation impinging upon saiddetector and thereby indicative of said property.
 24. The method ofclaim 19 comprising the additional step of conveying said detector alongsaid borehole by means of a wireline.
 25. The method of claim 19comprising the additional step of conveying said detector along saidborehole by means of a drill string.